The Surprising Motion of Ski Moguls

Ski moguls form on virtually all ski runs that are not mechanically flattened with grooming equipment. Those amazingly ordered structures are not planned or constructed; they organize spontaneously as a consequence of skiers turning and moving snow. Although phenomena that arise from self-organization are common, the moguls’ high visibility, ubiquity, and regularity make them a particularly surprising and impressive consequence of such seemingly random actions as ski turns. Ostensibly, skiers can turn when and where they please. Moreover, a skier’s turning radius depends on a variety of factors, including ski length and shape, snow conditions, skier ability, and the details of the skier’s knees and legs, which act as damped springs with a characteristic frequency. Nevertheless, the independent acts of many skiers form rows of moguls that not only space themselves in a regular checkerboard pattern (see panel a of the figure) but also migrate over time. And, although skiers invariably push snow down the mountain, the ski moguls move uphill.

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Here is a pretty cool time lapse of mogul movement.